Procedures of pipeline construction include methods of electric arc welding such as manual metal arc welding, semi-automatic welding by the flux cored wire, automatic submerged-arc welding or gas-shielded welding by the full wire.
Butt welding methods may include several runs with the help of beads. At this, each pass is made in a single run. Starting from the 3rd run each pass is made in two or more runs via individual pass. The number of passes is unlimited and depends on the thickness of welded elements (e.g., pipes to be welded). A drawback of this method is that welding of each pass in a single run leads to significant deformations, both welding and post-welding ones. It leads to hot (polygonizational) and cold cracks. Cracks are caused by the fact that the previous pass is cooled down before the next pass is made. Stresses during cooling of the non-uniformly heated metal are concentrated in the weld metal that leads to the loss of plasticity and crack formation.
During high-strength steel welding, mechanical features mostly depend on the value of heat input to the metal or the welding energy. In particular, the high heat input decreases strength properties of welded joints. To provide adequate mechanical features in weld areas, it is necessary to limit the maximum heat input value. The harder this limit is, the stricter requirements to the certain steel are. So, during development of welding technologies for high-strength steels, the main criteria for the high-quality welded joint is the optimal selection of parameters for the thermal welding cycle.
Reasons that prevent obtaining of the technical result that is provided by the method disclosed herein include an absence of requirements for the selection of parameters of the thermal welding cycle. Some known methods allow monitoring of the welding parameters. For example, method for pipe welding by the arc-welding current may include a controlled wave shape of welding cycles with short circuits. As a result, the high quality of the seam is provided (patent RU No. 2193478, published on Jun. 10, 2002, IPC B23K31/02). A drawback of this method is that the high-quality seam may be obtained only for the root pass and the metallurgical quality of metal remains at the same level for filling passes. This method helps to obtain the high-quality root penetration or to obtain the reverse mould in the root pass, i.e. geometrical parameters of the seam are high but this method has no effect on mechanical features of the weld metal.
There exists a method to determine the allowed modes for arc welding of austenitic steels and alloys via evaluation of the metal's propensity for inter-crystalline corrosion cracking. The method consists of the preliminary bead welding at different welding heat input on solid or composite plates with the certain correlation to the rate of welded joint cooling. Bead welding may be done by the electric arc, the heat input of which is 30-50% higher than the calculated one. This invention is aimed to increase the quality of welded joints of the equipment and pipelines made of austenitic steels and alloys (patent RU No. 2187091 published on Oct. 2, 2002, IPC G01N17/00). A drawback of such welding method is that welding modes allowed for austenitic steels are not allowed for welding of ferrous metal elements.
Another method is used for welding of cast iron products. The method pertains, in particular, to the arc welding of cast iron products with spherical graphite and can be used for welding of products made of the tubing stock in the oil and gas industry, energy industry, ship building and the communal service. Products are welded by the electric arc welding by the non-consumable electrode with the use of the filling wire that contains 55-95% of nickel. After the last filling pass is welded on both sides of weld, they weld technological bead with the width that is higher than thickness of welded products and the welding depth of no less than 0.35 of this thickness, at this technological beads and the last filling pass form the cap weld. This method provides for the higher strength of the welded joint, because stress raisers are excluded from the area of the weld's temperature effect due to technological beads (Patent RU No. 2098247, published on Dec. 10, 1997, IPC B23K9/23).
However, this method is allowed only for cast iron welding. Welding materials that contain 55-95% of nickel cannot be used for welding of low-carbon and low-alloyed construction steels because high-strength structures of the martensitic class prone to crack formation are formed on the weld line. In this case, it is impossible to obtain the full-strength welded joint.
Another method uses electric arc welding with a consumable electrode and pulse-type current modulation. Welding is made by the modulated current, where regulation of the pulse lengths and pauses in the welding current is made separately and independently. Additional pulses are applied with the range equal to the range of main pulses and the currency of no less than 50 Hz and the duration within 0.5-2 ms on the small welding current of 5-30 A during the pause interval. The heating power of the welding arc is controlled automatically at will of a welder by the change of parameters of main pulses in the function of average voltage fluctuation from the preset one by the small change of the 2 . . . 2.5 V arc space length. The technical result of the invention is the decreased radiation contrast of the arc in the pulse and the pause, the increased quality of the welded joint, the opportunity to manage the heat power of the arc upon the welder's wish, the increased mapping of the seam (Patent RU No. 2268809, published on Jan. 27, 2006, IPC B23K9/095). A drawback of this method is the absence of any requirements to welding of certain steels. The invention only gives methodical approaches to management of the welding arc power.
Another method for electric arc welding by a consumable electrode can be used for the pipeline welding. A flat isolated electrode is fixed with the output in the joint between welded pipes. The electrode is made as a part of the ring with the medium radius equal to the radius of welded pipes. Its width is equal to thickness of pipe walls. The electrode is bypassed by the highly conducting bus line through easily fusible tie plates. The electric arc is driven between the electrode and welded pipes. They set up the current and voltage values on the current source according to the required power determined upon the formula. They fuse the mandrel and butts of pipes. The power value is determined upon the formula depending on the needed value of the metal vapors gage pressure. The electric arc is automatically and spontaneously moved along the butt of the mandrel. The fused metal of the weld is cooled down. Parts of the joint that remain unwelded are to be welded by the consumable stick electrode. The electric welding device contains the flat consumable isolated electrode with the output. The invention allows to simplify the pipe welding process and to increase its quality (Patent RU No. 2119416 published on Aug. 27, 1998, IPC B23K9/14). However, this welding method is labor-intensive and the quality of the weld is unstable regarding to both mechanical features and the defect structure of the seam metal along its section.
Another method uses automatic argon-arc impulse for welding of steel pipes by a non-consumable electrode. At first, the pipe butts are opened up with formation of ring bevels on internal surfaces of butts and then the pipes are joined. The welded edges are moved along the electrode by the stepwise axis rotation of joined pipes with the rate of 1-1.5 rpm. About 2-5 s before starting rotation of pipes the arc is struck to heat the weld area. Welding is done by the torch installed downward at an angle of 60-85 grades to the vertical of the pipe axis. The arc current load is supported within the range of 110-155 A with an impulse duration of 0.7-0.9 s. The current in the pause is supported within the range of 10-30 A with the duration of 0.5-0.7 s. Stripping of the seam is supported with the range of 10-25 mm. That will allow to avoid crack formation in the pipe weld with coupon bending for the angle of more than 160 grades (patent RU No. 2262424 published on Oct. 20, 2005, IPC B23K9/167). However, the non-consumable electrode welding method is effective for welding of the seam's root run, thin-walled elements (with wall thickness of up to 4 mm) and especially of non-ferrous metals. It is virtually impossible to obtain the high-quality welded joint from the elements with the high wall thickness upon the strength uniformity criterion. Moreover, the use of this method increases the labor intensity of the process.
Another pipe butt welding method is used during manufacture of pipelines with both large and small diameters. The method includes opening up and blunting edges of welded details. The bluntness value is selected according to the steel grade, thickness of welded pipes, the heat input value during the electric arc welding and the speed of pipeline construction according to the correlation d/s<0.5, wherein ‘d’ is thickness of bluntness, and ‘s’ is wall thickness of welded pipes. The root weld is made by the electric-contact welding with fusing. The remaining part of opening is made by the electric arc welding. As a result, the high speed of pipeline construction is provided, and the labor productivity during installation and welding works is increased, the number of service staff and the expense of welding materials at the high quality of the welded joint (patent RU No. 2229968, published on Jun. 10, 2004, IPC B23K31/02). However, this welding method does not allow to have high-quality welded joints upon the resilience criterion. Resilience values are much lower than the values stated by regulatory documents. Moreover, the welded joints made by the contact welding, cannot be controlled using traditional non-destructive testing methods—ultrasound and radiographic methods.
It would be desirable, therefore, to overcome the drawbacks of these methods while retaining their advantages, by developing a new method for butt welding of elements, for example pipes used to construct a pipeline.